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1 Homberg s Chemistry: a Certain Truth into a Disputable Physics 1 Luc Peterschmitt; Rémi Franckowiak * In 1702, Homberg began publication of his Essays of Chemistry, which were supposed to be the first part of a whole book on chemistry. He began the first essay by giving the definition of chemistry as: Chemistry is the art of reducing bodies into their principles by the fire and of composing new bodies by blending different matters. 2 Such a definition may appear paradoxical: why did Homberg give, three years after the renewal of the Académie Royale des Sciences, such a classical definition of chemistry? 3 Indeed, he clearly recalled the Paracelsian spagyry, the art to join and separate. In fact, this definition was more important that it would have been, if it were only a way to recall a tradition. This definition has to be understood in the context of the end of the 17 th century and the debates about the rules of scientific knowledge. This definition was a way for Homberg to achieve a double task: a) He intended to give to chemistry a theoretical background: it was not only an experimental physics (role to which Fontenelle for example seemed to confine it), in order to make of the mechanical principles (motion, figure, size of particles) but the explanans of real scientific explanations; b) Homberg inverted the hierarchy between mechanics and chemistry: the latter only was certain, whereas the former was hypothetical, or, to use his own words, was not unquestionable. This double task was performed by a subtle use of the principles, which articulates the mechanical principles of physics and the chemical principles. Homberg wanted to show that chemistry was the certain part of a disputable physics: it will be shown that there was no paradox here. First, it is necessary to examine Homberg s commentary on his definition of chemistry. This commentary does not mainly bear on the notion of chemistry as an art, * UMR Savoirs, Textes, Langage (CNRS/ Universités de Lille 1 et Lille 3, FRANCE). 115

2 LUC PETERSCHMITT; RÉMI FRANCKOWIAK even if it was important as will be shown, but it bears on the notion of principle. Homberg distinguished two sorts of principles. In the most general sense, the principles of chemistry were its foundations: these principles were the arrangement, motion and size of the first matters it is important to note the plural here: it is not question of the universal matter of the Cartesian mechanism. However Homberg refused to discuss these principles: it would be, he says, too long, because, we do not have success in establishing nothing which is not incontestable. The principles of the second sort are the chemical principles: they are more material and sensible, and they set up the certain knowledge which was sought. At the end of this quick commentary, the inversion was achieved: physics was declared to be only hypothetical (as if everybody would easily agree), and its explanations will be, only likely. This did not mean that physics was false. This would be a contradiction: if physics was false, it would not be possible to say that a part of it was true. Moreover, Homberg recurrently declares that he is a physicist. But each time he says this in a peculiar context, it was a way to underline a way of reasoning: This reasoning, even if it seems likely, does not prove anything, unless it is sustained by well observed facts, because in physics we are so little clear-sighted that we are often mistaken, even when we believe to possess good facts and reasons. 4 To stress the limits of physics was a way to call on new or at least other experiments which was the task of chemistry. This means that, in short, it was not possible to be a good physicist without being a good chemist. But if chemistry was an art, as Homberg recalled in his definition, then one has to admit that it was not reducible to physics. It was not only that experiments were necessary. In this case the possibility that chemistry would be only an experimental physics would remain. In fact, chemistry had its own principles because it was an art of experimenting: its principles have to allow it to discover and expound facts: As chemical physics, which consists only in experiments and exposition of facts seeks only certain truth, it has established this second sort of principles, more material and sensible, by men of which it pretends to explain easily and in its way it own operations and to know thereby more distinctively the bodies it examines in its analyses. 5 The principles of chemistry have a certainty of which the principles of physics are deprived: they are supposed to derive from experiment. However, what was the point to say that chemistry was a part of physics? Sure, it was not possible to pro- 116 Neighbours and Territories: The Evolving Identity of Chemistry

3 Homberg s Chemistry: a Certain Truth into a Disputable Physics duce a true chemistry in a false physics; but how was it possible to pretend to have a certain chemistry inside a disputable physics? This is even more paradoxical when we remember that, during the 17 th and 18 th centuries, to adopt a mechanist position often leads, if not always, to dismiss chemistry as a real science. 6 In order to understand the sense of the reference to the physical principles, it is necessary to examine more closely Homberg s discourse about the chemical principles: Sulphur, Salt and Mercury. The first principle was Sulphur, or the matter of light: to Homberg, it was the only principle that was really active: I call Sulphur the active principle, because it acts alone and makes the others act. 7 Let examine how Homberg interpreted this activity: To make this opinion intelligible and likely, it is necessary to conceive first that the matter of light is always acting, which seems to me to be a necessary attribute of the active principle. Secondly, that this matter can get by itself into the other principles, change their figure, increase their weight and volume, and join them differently to produce all the mixed bodies which can be sensed, which the property we give to our Sulphur principle. 8 At a first glance, Homberg interpreted the activity of Sulphur mechanistically. This meant mainly that Sulphur was always moving, and modified the composition of bodies according to its motions. However, this motion seems to have no cause other than itself, at least this question of the origin of this continuous motion was not a chemical question (Homberg just suggests that its cause was divine). However, this motion was only a necessary property of the active principle : 9 it was not sufficient to understand the activity of Sulphur. Indeed, it did not explain how the Sulphur can join the other principles and so on. To explain that Sulphur can modify the chemical nature of the other principles, Homberg endowed it with a non-mechanical property: the natural gluten (which describes this property of gluing). The fact that this position seems to originate in the beginning of the 18th century renders it even more significant: the mechanical principles cannot explain by themselves the activity of Sulphur. Sulphur was a plain chemical principle, and not only a name for a body that can be accounted for in mechanical terms. The second principle was Salt. 10 As pure Salt, it was invisible, seen only when it was joined with another matter. More than of Salt, one should speak of salts, in the plural, which were only what was knowable with certainty. However, Homberg supposed, from 1708, that pure Salt was a sharp point and this figure accounted for the way in which salts act. However, the Salts were differentiated 117

4 LUC PETERSCHMITT; RÉMI FRANCKOWIAK from each other by the sulphur, which was always joined to them. Sulphur makes of an imperceptible point, an empirical chemical salt, which can be subjected to experiments: The difference between salts consists only in the different sulphurs which accompany them. 11 Moreover, Sulphur was what makes the salts act: All the actions of acids on alkalis and alike are only performed because they are pushed the one into the other by the matter of light, of which I have proved elsewhere that it was always moving, that it hits the solid parts of all bodies, that is to say that it pushes them continually. 12 Sulphur moves the salts, and their motion was determined by the association between the pure Salt and the peculiar sulphur that was joined with it. Sulphur makes of Salt a chemical object, of which Homberg gave a physical account. The last principle with which Homberg dealt was Mercury. It was said to be a principle only because of the failure of its chemical analysis: the principle was confused with common quicksilver. However, to Homberg, it was a principle of metals. The memoirs he devoted to Quicksilver illustrated this position, by proving the composition of gold as Sulphur and Quicksilver. Basically, Homberg pretended to have produced gold by introducing light (that was to say Sulphur) into a prepared quicksilver. 13 What is of interest was the way in which Homberg explained his experiment. The action of the matter of light consists in changing the nature of the quicksilver: it modifies its form, or its figure, or its figure, weight, volume. 14 Once again, the explanation was mechanistic: the matter of light destroyed the spherical particles of quicksilver, so that it became a new metal. Nevertheless, Homberg thought that his explanation was true: All what we have said of the destruction of gold and silver being true, that is to say that the important quantity of solar rays coming from the burning glass drove out the matter of light which stopped before in the little channels of the balls of quicksilver. 15 This reasoning or analysis was a model to understand the production of all sulphurous matters. Homberg constantly called on mechanism. In addition, one may ask, what does remain to chemistry as a part of physics? If chemistry was a part of physics, it was to be understood that it was a sort, or a kind, of physics. The specific difference is well illustrated by the natural gluten of the Sulphur, as a non-mechanic property, and certainly, natural also means essential. With this property, Sulphur links physics and chemistry. This was shown by Homberg s reflections on Salt and Quicksilver: chemistry deals with facts, physics tries to account for them, when no chemical discourse was possible, it was not possible to explain chemically the principles of chemistry, even if the physical explanation 118 Neighbours and Territories: The Evolving Identity of Chemistry

5 Homberg s Chemistry: a Certain Truth into a Disputable Physics was not absolutely complete. At least, all this means that the chemical bodies were subject to the physical laws. It is now possible to understand the signification of Homberg s so called mechanism: why did he call on mechanism and the principle of physics, and declare at the same time that they were at best, likely only? First, it is necessary to note that Homberg was not a mechanist, as the Cartesians had been; and he was neither a chemist who tried to elaborate a solution of compromise, as Marco Beretta put it, 16 between chemistry and mechanism. However, the mechanical principles gave an interpretive scheme for chemical phenomena, or at least for some of them. This scheme was probable; as already said, this does not mean that it is false. Such principles may be true, but there is no certainty about any mechanical explanation. That is why they do not constitute a veritable science. However, the question remains: why did Homberg not content himself with the certainty of facts and exposition of facts? A first evident answer, is that he needed explanations to produce a science. Moreover, dealing with principles lead him to a logical problem: it was not possible to explain chemical principles with chemical principles, of course. It is a signification of the mechanical scheme used by Homberg. More generally, it is to be noted that if experiments provide certainty, they are not intelligible immediately or by themselves, that to say that facts are to be explained. The problem is that the action of the principles cannot be observed directly: we can only perceive the effects of their action. This entails that, if the experiment is certain, the detail of what happens during the operation is not that clear (it could be said, it is completely unclear, after all, nobody is in the retort to see what happens here). Using mechanical principles makes of this unperceivable detail of experiment an object of thought, mechanism gives a rigorous conceptual representation of the operation. The experimented fact receives a kind of additional authentication, if it can be explained by the mechanical principles, then it is compatible with the general laws of physics. This gives reasons to think that, for example, it is not an experimental illusion. Finally, it is not impossible to produce gold, even if it seems to be quite incredible. To say it in another way, the physical or mechanical explanation does not account for the fact itself, but it accounts for its possibility. To do that, a probable, or a likely explanation is enough. However, it does not entail that the mechanical principle can explain the chemical fact. In this sense, Homberg is not a mechanist, and in spite of appearances, his chemistry was not mechanical, because it cannot be reduced to physics: a) Chemistry is certain, whereas the physical explanations are probable: they do not produce knowledge. In addition, if physical explanations bear on the pos- 119

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